This game's most important feature is its impressive character creation abilities. As a tool in every RPG game, this has seen constant development and interesting new looks in most every game that it is used in. Arcanum, in my opinion, has yet to be beaten by another game in its superb consideration of nearly all types of players and catering to them all without a specific class system.
Also, an interesting and genuinely intriguing tool that the game's developers inserted into the game was the rising conflict between Magics of the past and the rising Technologies in the world of Arcanum. Escelating into a miniature Cold War, Magic wielders would sabotage the trains in cities while the followers of technology would use these conflicts as an excuse to limit all magic users' freedoms with their powers. This, I found, led into an intriguing study of what "magic" means to people, even in the world outside of the game, and what technology meant to people, likewise in and out of the game.
Being a plot driven game with extreme freedoms, Arcanum might have been a little before its time and now has been mostly forgotten with its parent company declaring its bankruptcy and closure. A classic, nearly perfect example of the ideal for RPG games of the late 90's, the style in which Arcanum was made was both practical and brilliantly presented; a true classic of its time.